1. Field of the Invention
The present invention relates to an apparatus and method for detecting synchronization in a wireless communication system. More particularly, the present invention relates to an apparatus and method for detecting ranging signals.
2. Description of the Related Art
In general, voice services have been a primary concern in the development of communication systems. As communication technologies have advanced, voice services as well as various multimedia and data services are becoming increasingly important. However, voice-based communication systems have failed to satisfy user demand for multimedia and data services due to a relatively small transmission bandwidth and expensive service fees. Moreover, the development of communication industries and the growing demand for Internet services have resulted in an increased need for a communication system capable of effectively providing Internet services. Accordingly, a broadband wireless communication system has been introduced to effectively provide broadband Internet services.
In general, the broadband wireless access communication system uses an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA) scheme to achieve high-speed data communication when a physical channel signal is transmitted using a plurality of sub-carriers. A wireless access scheme of the broadband wireless communication system is being standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.16 group, which is one of the international standardization organizations.
Ranging is one of several random access processes defined in the broadband wireless communication system. Ranging can be classified into several categories, such as initial ranging, periodic ranging, bandwidth request ranging, handover ranging, etc. Initial ranging, in particular, is used to detect a propagation delay in order to adjust frame synchronization (or time synchronization) of a user device attempting an access to a network.
An initial ranging signal is a signal used when the user device in a particular cell accesses a communication system of the cell. The initial ranging signal is used to align a start point of a frame between the system and the user device by correcting a propagation delay parameter that can varies according to the location of the user device. When using a time division duplex (TDD) system, it is required to achieve uplink and downlink synchronization within a reference time period.
If initial ranging detection is unsuccessful, or if detected signal information has errors, a user device attempting access to a cell cannot receive communication services or may cause interference with another user device due to incorrect frame synchronization. That is, latency of a mobile station (MS) and deterioration of reception throughput may lead to degradation in overall performance of a communication system.
FIG. 1 generally illustrates detection of timing offset during initial ranging. For convenience of explanation, an IEEE 802.16-based ranging signal structure is used as an example. However, other ranging signal formats may be used without departing from the scope of the present invention.
As shown in FIG. 1, during initial ranging, an MS (e.g., a user device) repeatedly transmits a randomly selected ranging code during a detection period corresponding to two OFDM symbols 100. A sample offset corresponding to a cyclic prefix (CP) 102 is provided between a first OFDM symbol and a second OFDM symbol in order to deal with a multi-path channel.
In general, a cell radius for purposes of detecting the initial ranging signal is about several kilometers (km). A timing offset within a length of one OFDM symbol has to be estimated for an initial ranging allocation period. A transmit transition gap (TTG) 110 is provided between downlink and uplink transmissions in a TDD system. The TTG 110 corresponds to a time period in which a base station (BS) can switch from a transmit (Tx) mode to a receive (Rx) mode. During the TTG 110, the BS allows an antenna to switch to receiving data (Rx mode) instead of transmitting data (TX mode) and also allows a receiver to be in an operational state. A TTG value is determined according to a round trip delay (RTD) and a subscriber station receiver-transmit turnaround gap (SSRTG). The RTD depends on a distance between the BS and the MS. The SSRTG is an MS switching period 108 corresponding to a Tx/Rx switching gap of the MS.
As shown in FIG. 1, for an early arriving sample 104, a time offset detection period 114 starts earlier than an initial ranging reference timing 112 by a predetermined number of samples and has a length enough to estimate a timing offset within one OFDM symbol. As in the case of the early arriving sample 104 and a late arriving sample 106 shown in FIG. 1, detection of time offset is possible when an OFDM symbol for transmitting a ranging code of a user device is delivered within the time offset detection period 114.
Table 1 below defines a TTG/RTG period required for uplink/downlink switching within one frame according to bandwidth and shows a profile adopted in the IEEE 802.16d/e standard.
TABLE 1CellSamplingCPSymbolNo. ofRadiusBandwidthFFTFrequencyDurationDurationSymbolTTGRTGLimit  10 MHz102411.2 MHz11.2 μsec102.9 μsec47105.7 μsec60 μsec8.4 km 128 samples 1152 samples8.75 MHz1024  10 MHz12.8 μsec115.2 μsec42 87.2 μsec74.4 μsec  5.6 km 128 samples 1152 samples  7 MHz1024  8 MHz  16 μsec  144 μsec33  188 μsec60 μsec20.7 km  128 samples 1152 samples
According to Table 1, a maximum cell radius is about 8.4 km for a 10 MHz bandwidth, for example. For initial ranging, the timing offset detection period may be set to one OFDM symbol time of about 91.4 microseconds (μs). When timing offset detection is performed in the range of about −512 samples to about +511 samples with respect to early and late arrivals, the maximum cell radius is about 15 km.
As broadband wireless communication system spreads across the world, service regions may be considered differently from one service provider to another. In addition, with the diversification of targets of services to be provided, a problem arises in that a region to be covered may have a much larger radius than a typical cell radius. For example, with the introduction of a wide-area cell BS system in which one BS has to cover an area in the range of about 20 to about 30 km, there is a need for a method and system capable of operating such wide cell system without errors under a standard. The wide cell system can generally operate normally when the initial ranging can correct MS frame synchronization that differs significantly from a BS frame.
Typical method and system of detecting the initial ranging signal has some of the following problems. For example, typical ranging detection algorithm detects a timing offset within a range of about −512 to about +511 samples with respect to a 1024-Fast Fourier Transform (FFT) as shown in Table 1 above. If a cell radius is large, an initial ranging signal from the MS may be delivered before −512 sample timing (i.e., early arrival) or after +511 sample timing (i.e., late arrival). In this case, ambiguity for the timing offset occurs. Accordingly, there is a need for a method and system capable of correctly detecting a timing offset in a BS when a delivered ranging signal is not within a given detection period.